The majority of his recent papers deal with the development and application of chemical transport models (CTM) to studies in regional atmospheric chemistry, air quality and climate. These research activities include the development of comprehensive air quality models and their application to regional and international air pollution problems. His studies have led to a greater appreciation and understanding of the importance of long range transport of pollutants within Asia and across the Pacific. His work has also explored the importance of dust on atmospheric chemistry, and this work has helped to stimulate laboratory as well as large-scale field experiments. His research has involved the development of innovative modeling tools, including techniques to optimally integrate measurements and models via formal chemical data assimilation. Most recently his work has focused on the role of black carbon (BC) in the atmosphere and its dual role as an air pollutant and climate warming agent. He is a member of the scientific steering committee for the UNEP ABC Asia project, where he has published recent papers on the important role of BC in the climate system. He recently was a co-lead author on a UNEP study aimed at providing a critical assessment of the role of BC as a short lived radiative forcing agent. He also serves as chair of the Scientific Advisory Group for the World Meteorological Organization Global Atmospheric Watch Urban Meteorology and Environment project, which is focused on building capacity worldwide to improve air quality forecasts and related services. He is an active instructor and advisor, having supervised 33 MS and 31 PhD students.